The present invention relates to centrifugal clutches such as are used in light vehicles, chain saws, pumps, wind mills, compressors, engine, electric motor, wind driven products and the like for variably coupling rotational power sources to driven loads.
Centrifugal clutches are disclosed, for example in U.S. Pat. Nos. 2,942,711 and 3,971,463 to the above-named inventor, which patents are incorporated herein by this reference. Typically, a plurality of radially movable shoes frictionally contact an inside surface of a drum that rotates concentrically with a hub to which the shoes are connected. Normally, the hub is fixed on a driving shaft or face plate that is coupled to an engine or power source, the drum being coupled to a driven load by means such as a belt, chain or direct coupled drive. Such clutches transmit little or no torque at low speeds, and progressively increased torque at higher speeds in proportion to centrifugal forces to which the shoes are subjected. In the above-identified patents, spring biasing elements are employed for maintaining the clutches in a disengaged condition at speeds below a predetermined threshold for permitting the engine or other power source to idle at no load.
Centrifugal clutches as previously implemented are subject to a number of problems and disadvantages. For example:
1. They are ineffective in that they fail to provide a desired torque-speed relationship to match the characteristics of the power source; PA1 2. They are short-lived in that there is excessive slippage and consequent wear of the shoes, particularly when there is a large vibrational component or high inertia load in the torque; PA1 3. They are unreliable in that springs used therein are subject to breakage, particularly in that they have fatigue inducing stress risers in hooks at opposite ends; PA1 4. They are unsafe in that the shoes can fly apart in case there is structural failure of the drum; and PA1 5. They are undesirably expensive to produce. PA1 1. At start up and with the engine revved up the clutch pedal is gradually let out. A smooth get-away is accomplished, but at the expense of rapid wear and short clutch life. PA1 2. At start-up and with the engine revved up the clutch pedal is abruptly let out, resulting in a jerky forward motion and/or stalling of the engine. PA1 1. It can be pre-set for optimum performance in each application; PA1 2. There is increased customer satisfaction; PA1 3. There are reduced warranty and liability claims; PA1 4. Tooling and design staff requirements are reduced; PA1 5. Both production and service inventory requirements are reduced; PA1 6. New applications for the clutch can be serviced more quickly; and PA1 7. Profitability extends to vast markets not now being served. PA1 1. There is reduced shock to the overall system; PA1 2. Operation is cooler, and with longer life; PA1 3. Overall product performance can be fine-tuned, with separate speed-sensitive and torque-sensitive attributes; and PA1 4. There is controllable over-load protection.
The clutch as disclosed in the '711 patent can be assembled in a variety of ways to selectively provide correspondingly different torque-speed relations such as straight-centrifugal with selectable speeds of engagement; relatively quick-acting; or relatively smooth-acting. However, none of the torque-speed relations is entirely satisfactory, and there are numerous possibilities for incorrect assembly. The clutch as described in the '463 patent provides driving member projections that are successively engageable with the shoes at increasing speed increments for progressively increased torque transmission. Again, none of the torque-speed relations is entirely satisfactory, particularly when it is desired to obtain initial engagement at a high percentage of the full-load speed of the clutch, and the clutch is excessively complex and expensive to produce.
It is also known to have direct metal-to-metal contact between the drum and metal shoe members of centrifugal clutches. However, the applications for such clutches are severely limited by a lower coefficient of friction, excessive wear, and galling.
U.S. Pat. No. 5,560,465 to the present inventor discloses a centrifugal clutch having an interlocking combination of shoes and compression biasing springs for preventing separation of the shoes in the event of axial disengagement or structural failure of the drum, and a spring biased wedge that smoothly transitions between low-gain and high gain orientations within a narrow speed range. Unfortunately, the clutch described in the '465 patent is not entirely satisfactory in that the drum can become damaged by direct contact with metallic shoes in the event that lining material on the shoes is worn through. Also, desired gain-speed profiles typically desired in particular applications are difficult to obtain, particularly without requiring undesirably large inventories of variantly configured components.
Thus there is a need for a centrifugal clutch that provides desired torque-speed profiles that smoothly transition from no-load to full load in an adjustable narrow speed range, that is fail-safe against lining wear-through as well as against separation of the shoes in case of failure of the drum, and that is reliable, long-lasting, and inexpensive to produce.